1. Field of the Invention
This invention relates to a method of and apparatus for obtaining a radiation image, and more particularly to a method of and apparatus for obtaining a visible radiation image by use of a stimulable phosphor which is stimulable with visible light or infrared rays. The stimulable phosphor used in this invention records a radiation image in the form of a stored energy of a radiation upon exposure to the radiation and emits visible light when stimulated thereafter by visible light or infrared rays.
2. Description of the Prior Art
Heretofore, in order to obtain a radiation image there has been used a so-called radiography employing silver halides. Recently, due to the problem of shortage of silver resources, there has been a great demand for a radiation image recording system which does not use silver halides.
As one of the radiation image recording systems, a new method of radiation image recording is disclosed in U.S. Pat. No. 3,859,527. The method disclosed therein employs a stimulable phosphor which emits light when stimulated by visible light or infrared rays after exposes to a radiation such as X-rays, .alpha.-rays, .beta.-rays, high energy neutrons, electron beams, vacuum ultra-violet rays, ultra-violet rays and other similar electromagnetic wave or corpuscular beam. In this method, a radiation image conversion panel is first exposed to a radiation having an image pattern and stores the radiation energy and then exposed to stimulating rays like visible light or infrared rays. Upon exposure to the stimulating rays, the phosphor emits light in accordance with the stored energy of radiation as fluorescent light. The light emitted upon stimulation is detected by a detecting means to visualize the radiation image.
The above described radiation image recording or visualizing system is very promising as a radiation image recording method replacing the above mentioned conventional radiography. However, in the above method utilizing a stimulable phosphor there is a defect in that the image is degraded by a noise and the sharpness is lowered when the radiation image conversion panel is repeatedly used many times or the panel is left to stand for many hours after once used. This is because it sometimes happens that the panel receives radiation energy of other radiations than the radiation used for recording the radiation image before the panel is subjected to exposure to the latter radiation, and that the stored energy of the other radiations emits light together with the energy of the latter radiation. In other words, when the radiation image conversion panel is let to stand for a long time, it absorbs the energy of radiation as of radiations emitted from radioactive isotopes like .sup.226 Ra and .sup.40 K minutely contained in the stimulable phosphor and the environmental radiations. The absorbed radiation energy is stored in the stimulable phosphor and causes the noise. Further, particularly when the panel is repeatedly used, the stored energy partly remains in the stimulable phosphor after the phosphor has been stimulated and emitted light in case that the stimulation is insufficient due to low intensity of the stimulating rays or improper wavelength of the stimulating rays. The remaining energy causes the noise in the subsequent recording process. The noise is prominently observed when the sensitivity of the radiation image conversion system which reads out the recorded image and converts the image into a visible image. The noise of course deteriorates the quality of the radiation image finally obtained and lowers the diagnostic efficiency and accuracy in case of handling a radiation image for diagnostic purposes. Therefore, there is a great demand for the improvement to eliminate the noise in the above described radiation image conversion or recording system.